Nanostructured materials are used to produce electrocatalysts from powders of a target composition prepared by mechanical alloying for example and they have a defined structure and morphology. For tests at the laboratory scale, the powders can be pressed into pellets of a geometrical surface of 2 cm2 for example.
A number of methods for preparing these powders have been developed in order to allow producing electrodes of sizes compatible with industrial requirements. For example, electrocatalytic coatings may be fabricated on the surface of substrates by thermal spraying of nanostructured metastable powders obtained by mechanical milling or other processes to yield nanostructured powders. In a thermal spray process, the powder is introduced into hot plasma so as to be heated (and possibly melted, either partially or totally) and directed at high speed towards the substrate for deposition. As a result of heat, the structure and composition of the powder are liable to be modified, and oxide layers may be created at the interface between the substrate and the coating being formed, which results in a weak adhesion between the coating and the substrate, which may ultimately cause delamination of the catalyst coating from the substrate during electrolysis or formation of oxides at the surface of the coating, in which case the electrodes need to be submitted to an activation step before being used.
During the thermal spray process, it is extremely difficult to control the surface conditions, such as for example oxidation, roughness, contamination, etc., of the substrate as the coating is being deposited, which may result in a reduced adhesion between the coating and the substrate and a weak interface between them. As a result, the coating obtained is found not to be stable and can be damaged under operating conditions.
Alternatively, a cold gas dynamic spray (cold spray) process has been developed, wherein a supersonic gas jet is used to accelerate solid fine powders of various materials above a critical velocity at which the particles impact, deform plastically and bind to the substrate to form a coating.
As illustrated in FIG. 1, the cold spray process basically uses the energy stored in a high pressure compressed gas, such as air, nitrogen, helium and mixtures thereof for example, to propel fine powder particles at very high velocities (500-1500 m/s). The compressed gas is fed to a spray gun and the gas exits through a nozzle at supersonic velocity. A high pressure powder feeder introduces the feedstock powder material into the high velocity gas jet. The powder particles are accelerated in the gas flow to high velocity, and only moderately heated. On impact with a substrate, they deform, by plastic deformation, and bond to form a coating. The particles remain in the solid state and are relatively cold, so bulk reaction on impact is limited to solid state chemistry. The process imparts little to no oxidation to the spray material, so surfaces stay clean, which promotes bonding. No melting and relatively low temperatures result in very low shrinkage on cooling. Moreover, due to high strain induced upon impact, the coating tend to be stressed in compression and not in tension, as typically occurs at the liquid/solid interface in most other thermal spray processes. Low temperatures also permit retaining the original chemistry and phases of the powders in the resulting coating, with only minor change due to deformation and cold working (see for example WO 2005/079209).
Sodium chlorate (NaClO3) is mostly used to produce chlorine dioxide for bleaching paper pulp, since it allows reducing by about 84% the emission of chlorinated materials compared to the use of elementary chlorine. The first step in the preparation of sodium chlorate (NaClO3) is the electrolysis of chloride ions (Cl−) from a brine solution that generates chlorine (Cl2). Chlorine is then converted to sodium chlorate through a series of chemical steps and recrystallized. Up to 70% of the total production costs of sodium chlorate are due to electric energy needs during the process. Electrolysis is responsible for almost 95% of the electrical consumption of the total process. That is a reason why efforts are developed to whatever efficiency improvement, which could allow reducing these electrolysis costs.
Nanostructured powders of Ti—Ru—Fe—O have been shown to be a good catalyst for the hydrogen evolution reaction in the process of sodium chlorate synthesis (see for example references 1-15; WO 2006/072169). Coatings prepared by thermal spraying were shown to be active. However, the stability of such coatings varies according to the size thereof. While small surfaces (1 cm2) show a good stability, typically of more than about 30 days for example, coatings on larger surfaces (500 cm2 and up) tend to peel off and their activity decreases after only a few days of operation.
Another application is, for example, aluminium electrolysis. Aluminium production by the Hall-Héroult process involves the electrolytic decomposition of aluminium oxide dissolved in a molten cryolite (Na3AlF6) bath operating at temperatures around 960° C. Molten aluminium is produced at the cathode and carbon dioxide is formed at the consumable carbon anode. The process requires a large amount of energy and produces significant emissions of greenhouse gases. A number of environmental and economic incentives are in favour of developing aluminium production technology with inert oxygen-evolving anodes: reductions of greenhouse gas emissions (CO2, CFx, etc.), cost reduction by eliminating the consumable carbon anode plant, capital saving in the smelter by enabling higher Al production per unit volume of cells, and a reduction in operating and labor costs by eliminating the frequent anode change operations, etc. However, several decades of research have shown that the development of inert anodes for Al production is not a straightforward task. Indeed, an inert anode material must satisfy multiple requirements including a low corrosion rate, typically below 10 mm/year, good resistance to fluorination and anodically produced oxygen, stable potential and low overpotential for oxygen evolution, low electrical resistivity, adequate mechanical strength and thermal shock resistance, simple electrical connection, low cost and ease in manufacturing on an industrial scale. In addition, the produced aluminium must have acceptable impurity levels for major applications.
Inert anodes made of nanostructured Cu-based alloys prepared by mechanical milling display a stable cell voltage, maintain their mechanical integrity and induce a limited pollution of the produced aluminium for a 20 h electrolysis in low-temperature (700° C.) KF—AlF3 electrolyte (see for example, references [15-18]). However, the fabrication of large and dense electrodes required for Al electrolysis at the industrial scale from these ball-milled materials remains a challenge.
There is a need for coatings and electrodes that meet the requirements of the industry and are more efficient.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.